Most studies of major depressive disorder (MDD) pathophysiology have examined a single, putative causative domain. In this proposed study of MDD pathogenesis, we will evaluate relationships between three path- ogenic domains related to MDD, that are largely unknown because these three domains have never been studied together in a single depressed population. The domains are: neuroinflammation, serotonergic dysregulation and mitochondrial dysfunction. This project is a cross-sectional study that will directly assess brain functioning in all three pathophysiologic domains by using positron emission tomography (PET) to measure neuroinflammation and 5-HT1A autoreceptor binding, and near infrared spectrometry (NIRS) to measure mito- chondrial function, in MDD (N=45) and healthy volunteers (HV; N=20). A third-generation, high-affinity PET tracer, [11C]ER176, will measure binding to translocator protein (TSPO), a marker of glial activation and mito- chondrial function. Brain mitochondrial function also will be assessed with transcranial near infrared spectros- copy (NIRS). PET with [11C]WAY-100635 will quantify 5-HT1A autoreceptors which regulate serotonin neuron firing and release. Brain measures will be compared between MDD and HV groups and within MDD as they relate to depression severity. Brain indices also will be correlated with peripheral blood measures, which will include kynurenine pathway components, a panel of cyto/chemokines, and the mitochondrial health index (MHI), a scalar measure of peripheral mitochondrial function. This rich dataset will be used to generate descriptive models of interrelationships between pathophysiologic domains and the relative correlation of each domain with depressive symptom severity. Exploratory analyses will seek to identify subgroups of patients who exhibit do- main-specific pathology. AIM 1: Compare 45 unmedicated, depressed, non-psychotic DSM5 MDD patients with 20 HVs, on neuroinflam- matory, serotonergic and mitochondrial effects measured in vivo in brain. Determine correlations of brain measures with MDD severity in the MDD group. AIM 2: In the same groups from Aim 1, determine the relation- ships of brain TSPO binding and oxCOX activity to the respective peripheral measures of neuroinflammation and mitochondrial functioning. blood cyto/chemokines, kynurenine pathway components, and the mitochondrial health index (MHI). AIM 3: Develop descriptive models. This study is innovative in combining assessment of neuroinflammation, mitochondrial function, and 5- HT autoreceptor binding in a single set of subjects, in using a third-generation TSPO radiotracer, and in employing novel NIRS and MHI methodologies. The research team has a strong record of translational re- search with all proposed modalities and is equiped to carry out this proposed study. Findings from this study would have potential to unify major theories of depression pathophysiology, and guide the development of new, more individualized treatment approaches.